Control rod drives (CRDs) are used to position control rods in boiling water reactors (BWRs) to control the fission rate and fission density, and to provide adequate excess negative reactivity to shutdown the reactor from any normal operating or accident condition at the most reactive time in core life. Referring to FIG. 1, each CRD is mounted vertically in a CRD housing 10 which is welded to a stub tube 8, which in turn is welded to the bottom head of the reactor pressure vessel 4. The CRD flange 6 is bolted and sealed to the flange 10a of the CRD housing 10, which contains ports for attaching the CRD hydraulic system lines 80, 81. Demineralized water supplied by the CRD hydraulic system serves as the hydraulic fluid for CRD operation.
As shown schematically in FIG. 1, the CRD is a double-acting, mechanically latched hydraulic cylinder. The CRD is capable of inserting or withdrawing a control rod (not shown) at a slow controlled rate for normal reactor operation and of providing rapid control rod insertion (scram) in the event of an emergency requiring rapid shutdown of the reactor. A locking mechanism in the CRD permits the control rod to be positioned at 6-inch (152.4-mm) increments of stroke and to be held in these latched positions until the CRD is actuated for movement to a new position. A spud 46 at the top of the index tube 26 (the moving element) engages and locks into a socket at the bottom of the control rod. Once coupled, the CRD and control rod form an integral unit which must be manually uncoupled by specific procedures before a CRD or control rod may be removed from the reactor.
When installed in the reactor, the CRD is wholly contained in housing 10. The CRD flange 6 contains a withdraw port 70 and an insert port 66 with an integral two-way check valve (with a ball 20). For normal drive operation, drive water is supplied via an associated hydraulic control unit (HCU) to the insert port 66 for drive insertion and/or to withdraw port 70 for drive withdrawal. For rapid shutdown, the check valve directs external hydraulic pressure or reactor pressure to the underside of drive piston 24. When higher than the external hydraulic pressure, reactor pressure is admitted to the two-way check valve from the annular space between the CRD and a thermal sleeve (not shown) through passages in the CRD flange, called scram vessel ports.
Referring to FIG. 2, the CRD further comprises an inner cylinder 57 and an outer tube 56, which form an annulus under a collet piston 29b through which water is applied to the collet piston to unlock index tube 26. A collet housing 51 (which is part of outer tube 56) is provided with ports 73 to permit free passage of water from the clearance space between the outer diameter of index tube 26 and the inner diameter of inner cylinder 57 and the inner diameter of collet housing 51. The bottom of collet piston 29b normally rests against a spacer 52 in the upper portion of the annular space. Grooves in the spacer permit the passage of water between the bottom of the collet piston 29b and the passage area within the cylinder, tube and flange.
The locking mechanism consists of collet fingers 29a, collet piston 29b, barrel 35, guide cap 39, and collet spring 31. The mechanism is contained in the collet housing 51 portion of outer tube 56 and is the means by which index tube 26 is locked to hold the control rod at a selected position.
The collet assembly consists of a collet piston 29b fitted with two expansion piston seal rings 28 and two contraction piston seal rings 27, six fingers 29a and a retainer (not shown) and is set into a bore in the collet housing 51. In addition, a spring 31, barrel 35 and guide cap 39 complete the components installed in the collet housing 51.
Guide cap 39 is held in place above the collet by three plugs 37 which penetrate the upper end of collet housing 51, and which are held in place by fillister-head screws. It provides a fixed camming surface to guide collet fingers 29a upward and away from index tube 26 when unlocking pressure is applied to collet piston 29b. Barrel 35 is installed below guide cap 39 and serves as fixed seat for collet spring 31.
The collet mechanism requires a hydraulic pressure greater than reactor pressure to unlock for CRD-withdraw movement. A preload is placed on collet spring 31 at assembly and must be overcome before the collet can be moved toward the unlocked position. For control rod withdrawal, a brief insert signal is applied to move index tube 26 upward to relieve the axial load on collet fingers 29a, camming them outward against the sloping lower surface of index tube locking notch 55. Immediately thereafter, withdraw pressure is applied. In addition to moving index tube 26 downward, this pressure is at the same time applied to the bottom of collet piston 29b to overcome the spring pressure and cam the fingers 29a outward against guide cap 39. When the withdraw signal ceases, the spring pressure forces the collet downward so that fingers 29a slip off guide cap 39. As index tube 26 settles downward, collet fingers 29a snap into the next higher notch and lock. When collet fingers 29a engage a locking notch 55, collet piston 29b transfers the control rod weight from index tube 26 to the outer tube 56.
Unlocking is not required for CRD insertion. The collet fingers are cammed out of the locking notch as index tube 26 moves upward. The fingers 29a grip the outside wall of index tube 26 and snap into the next lower locking notch for single-notch insertion to hold index tube 26 in position. For scram insertion, index tube 26 moves continuously to its limit of travel during which the fingers snap into and cam out of each locking notch as index tube 26 moves upward. When the insert, withdraw or scram pressures are removed, index tube 26 settles back, from the limit of travel, and locks to hold the control rod in the required position.
The drive piston 24 and index tube 26 are the primary subassembly in the CRD, providing the driving link with the control rod as well as the notches for the locking mechanism collet fingers. Drive piston 24 operates between positive end stops, with a hydraulic cushion provided at the upper end only. Index tube 26 is a nitrided stainless-steel tube threaded internally at both ends. The spud 46 is threaded to its upper end, while the head of the drive piston 24 is threaded to its lower end. Both connections are secured in place by means of bands 25, 25' with tab locks.
There are 25 notches machined into the wall of index tube 26, all but one of which are locking notches 55 spaced at 6-inch intervals. The uppermost surfaces of these notches engage collet fingers 29b, providing 24 increments at which a control rod may be positioned and preventing inadvertent withdrawal of the rod from the core. The lower surfaces of the locking notches slope gradually so that the collet fingers cam outward for control rod insertion.
When a control rod is driven upward to its fully inserted position during normal operation or scram, the upper end of the piston head (not shown) contacts the spring washers 30 which are installed below the stop piston 33. Washers 30 and stop piston 33 provide the upper limit of travel for drive piston 24. The spring washers, together with the series of buffer orifices 53 in the upper portion of piston tube 15, effectively cushion the moving drive piston 24 and reduce the shock of impact when the piston head contacts the stop piston.
A stud 59 is welded to the upper end of tube piston 15. Stud 59 is threaded for mounting the stop piston 33. A shoulder on the stud, just below the threaded section, is machined to provide a recess for the spring washers 30. The stop piston 33 provides the seal between reactor pressure and the area above the drive piston. It also functions as a positive-end stop at the upper limit of drive piston travel. Seals 34 include an upper pair used to maintain pressure above the drive piston during CRD withdrawal and a lower pair used only during the cushioning of the drive piston at the upper end of the stroke. Two external bushings 32 prevent metal-to-metal contact between stop piston 33 and index tube 26.
As seen in FIG. 3, spud 46, which connects the control rod 90 and the CRD, is threaded onto the upper end of index tube 26 and held in place by locking band 25'. The coupling arrangement will accommodate a small amount of angular misalignment between the CRD and the control rod. Six spring fingers 46a permit the spud to enter the mating socket 92 on the control rod. A lock plug 94 then enters spud 46 from socket 92 and prevents uncoupling. An uncoupling rod 48 positions the control rod lock plug 94 such that it supports (i.e., opposes radially inward deflection of) the spud fingers 46a when the control rod and CRD are coupled.
The uncoupling rod 48 is welded to the flared end of a tube 43 such that a dimension of 1.125 inches exists between the top of rod 48 and the top end of spud 46. This is a critical dimension and must be maintained to ensure proper CRD and control-rod coupling. For this reason, uncoupling rods cannot be interchanged unless the critical dimension is verified. Tube 43 is slidably supported in the base of the spud at the upper end of the CRD (see FIG. 3).
Two uncoupling mechanisms are provided. The lock plug 94 may be raised against the return force of a spring 95 by an actuating shaft 96. The control rod, with lock plug 94 raised, may then be lifted from the CRD. The lock plug may also be raised from below to uncouple the CRD from below the reactor vessel. To accomplish this, an uncoupling tool is attached to the bottom of the CRD and used to raise piston tube 15 and uncoupling rod 48. When the control rod is in its "full-out" position, i.e., backseated position atop the guide tube (not shown), drive piston 24 is separated from piston head 15b by a distance of 21/8 inches. Raising piston tube 15 and uncoupling rod 48 by 11/8 inches lifts lock plug 94 out of the spud. The drive piston/index tube/spud assembly 24/26/46 is then withdrawn until the drive piston sits on the piston head 15b (i.e., a distance of 1 inch), thereby disengaging the spud from the control rod coupling socket 92 (i.e., uncoupling the control rod). The uncoupling tool is then lowered by 11/8 inches to lower the control rod, assembly 24/26/46 and piston 15 together until piston head 15b is again backseated on the CRD ring flange 17.